If you don't tell them you'll always wonder "What might have been".
All you need to say at some point is, "I'm not sure when this began to happen but I'm having feelings for you in a non plantonic way. For the sake of our friendship I wonder if it would be better if we spent less time together." Be quiet and listen to his response.
If he tells you he is feeling the same way about you then you are off to the races. If he says you're probably right you should spend less time together then you have your answer. This will allow yourself to "give up on that dream" and gradually open your heart to the possibilty of "Mr. Right" coming along.
You'll know he's "Mr. Right" because he will automatically recongize that you are "Ms. Right"